High altitude fuel

Because of tank heating, fuel volatiUty is also more critical in supersonic aircraft. For example, the Concorde tank is pressurized to prevent vapor losses which could be significant at high altitude where fuel vapor pressure may equal atmospheric pressure. The tank can reach 6.9 kPa (1 psi) at the end of a flight. The need to deoxygenate fuel for thermal stabiUty in the HSCT will doubdess require a similar pressurized system. 

The oxygen sensor closed loop system automatically compensates for changes in fuel content or air density. For instance, the stoichiometric air/fuel mixture is maintained even when the vehicle climbs from sea level to high altitudes where the air density is lower. 

The 10%, 20%, 50%, and 90% distillation points are specified to ensure that a properly balanced fuel is produced. Vapor pressure is also related to cold engine starting and helps limit vapor lock at high altitudes. 

Fuel must flow during long periods at high altitudes. Fuel must not solidify and block lines, filters, and nozzles. Low-temperature viscosity must also be controlled to ensure that adequate fuel flow and pressures are maintained. Fuel viscosity can also significantly affect the fuel pump service life. 

Low-molecular-weight vapors can cause vapor locking a critical concern at high altitudes in aviation turbine fuels... 

Another possible solution to the problem of high temperature stability is the use of additives. Not exactly a stranger to petroleum people (as evidenced by use in gasoline and lubricants) they generally fall into two classes metallic and non-metallic. The former, for the most part are metal salts of sulfonates or naphthenates, whereas the latter are either amines or amine derivatives (later other organics may prove more effective) Use of additives in jet fuels, however, must of necessity be approached with caution. As surface active materials, many have a variety of uses and properties. Hence, they must not introduce new problems such as foaming at high altitudes, emulsification, or interference with low temperature flow. These could easily be severe limitations, but additives are under serious consideration thruout the industry... 

Conventional fuels such as firewood, coal etc. are not suitable at high altitudes because of difficulty in ignition, low heat output due to lower oxygen content and low ambient temperature. Gel-based fuels were not found very attractive because of their high degree of inflammability and toxic combustion products. 

Applications in enhanced combustion and life-support systems provide the majority of the demand for oxygen. It is used extensively in medical applications for therapeutic purposes, for resuscitation in asphyxia, and with other gases in anesthesia. Also it is used in high-altitude flying and deep-sea diving, and it is used for life-support and as a fuel oxidizer in the U.S. space program.1... 

Ballard assembles and tests stacks, to confirm their characteristics and optimise such water management problems as electrolyte humidity control and cathode water removal, via wettable materials in the porous electrode, to the external air flow. Potential difficulties, exclusive to the water-producing PEFC, can be seen with 100% humidity in the tropics, and with freezing conditions in cool climates. High altitudes can be difficult for all fuel cell types, via low oxygen density. 

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